Rewrite encryption

[uhthomas]

The current system read content fully into memory, and encrypts it using AES128-GCM. The key, IV and nonce are then stored in a URL hash. This is fine, but has a few down-sides:

• In-memory isn't great for large files (this is primarily because of browser limitations).
• The IV and nonce can be public, and don't need to be stored in the URL, making it shorter.
• AES128-GCM is fine, but ChaCha20-Poly1305 is the current standard for authenticated encryption.
• It's not easy to iterate on as there is no meta association. Writing a protobuf definition as a sort of header would allow the inclusion of extra metadata and flexibility in encryption ciphers.
• No support for passwords

A lot of these would be solved by adding a protobuf definition, but the big issue is browser support for:

• Fetch streams (read/write)
• File API